I am a pianist who has been learning the cello for 5 months (as a very late starter!) and I am making progress and enjoying it. It is certainly a challenge - fingers have to be in exactly the right position to get the correct note pitch, while aiming for a good tone and trying to keep the right hand in the correct position on the bow (haven't solved that one yet - my fingers keep falling off!). All this to think of while sitting properly, trying not to drop the left arm, keeping time (I find this more difficult on the cello because of allowing for different lengths of bow stroke) and reading the music. Still, you'll know about these aspects of learning from playing the viola, so you should have a head start there. The strings are harder to press down than viola strings.
If you have not played an instrument that uses bass clef (apart from piano) you might find that it takes getting used to (tenor clef too when you get a lot more advanced). I play bass recorder, so haven't had a problem with that, but I remember what it was like when I first played the bass recorder - you would think that knowing it for piano would mean you would be able to play it straight away, but it doesn't seem to work like that.
Make sure you get a really good specialist cello teacher if you do decide to learn - the early stages are very important as it is when your habits are set. Mine is really brilliant. She is a performer as well as a teacher, and very thorough.
I'm playing two of my easy beginner pieces at the Adult Learners Concert next Saturday (see Adult Learner thread), so wish me luck as I'll be quite nervous on the day - as we all shall be!
Good luck with your cello playing future.